The present invention relates to electromagnetic interference measurements (EMI) of digital systems and, more particularly, to determining electromagnetic compatibility (EMC) of integrated circuits (ICs).
Electromagnetic radiation emitted from a digital system is mainly at the fundamental frequency of its swiching operation and that frequency""s harmonics; for systems with multiple clocks, multiple fundamental frequencies and harmonics will be present. At other frequencies, system radiation levels are typically undetectable.
International as well as national regulatory standards exist which specify allowable levels of EMI emissions from unintentional radiators. The main purpose of these standards is to protect the radio frequency spectrum for equipment licensed to operate at appropriate frequencies. Because the regulated frequencies reach into the gigahertz range and the allowable signal levels are typically in the microvolt range, the test set-up when measuring EMI often influences the test results. Therefore, in addition to allowable signal levels, these regulations also specify measuring methods in an attempt to standardize the compliance tests and improve their repeatability. These factors increase both the length and cost of typical EMI tests.
Typically, EMI regulations directly apply only to full systems, such as a personal computer, and not directly to system components, such as video cards or microprocessors. An accepted industry practice, however, to test the EMC of system components is to use the xe2x80x9csubstitution methodxe2x80x9d. In this method, a compliant system is used, and an original component is replaced with a new component. If the system is still compliant, then the new component is determined to be electromagnetically compatible.
Because the microprocesor is one of the major contributors to system level EMI, EMC assessment of microprocessors is a concern of CPU manufacturers. This assessment encompasses two facets: verification of compliance to EMC standards and component characterization. When performing the substitution method to verify EMC compliance, multiple tests of multiple components on multiple systems are needed to provide any degree of confidence in the test results. Component characterization, the other facet of EMC assessment, identifies the effects that design choices have on EMI emissions. For purposes of characterization, a high number of package design options (e.g., package layer stack-up, chip-cap configuration, etc.) are evaluated and compared to arrive at valid manufacturing decisions.
One common feature of both facets is the large number of tests which need to be performed in order to properly assess a microprocessor""s EMC. The far-field tests typically used for radiated emission measurements are both time consuming and expensive and the component industry needs an alternative method to perform EMC assessment. The Society of Automotive Engineers in developing their standard SAE J1752-1, entitled xe2x80x9cEMC Measurement Procedures for Integrated Circuitsxe2x80x9d, has investigated using near field EMI measurement methods for ICs; but these methods typically involve specially manufactured test boards attached to a modified Transverse Electromagnetic Mode (TEM) cell and introduce measurement errors when operated above one gigahertz. Presently, an alternative EMC assessment method and arrangement is needed, which produces EMI data corresponding to a microprocessor or other IC with minimum influence from other components such as cables, power supplies, and which allows testing of multiple parts in a timely and economic manner.
The present invention addresses the need for alternative EMI measurement methods and arrangements for digital electronic systems, especially alternatives to far-field measurement requirements as described in regulatory standards on EMI.
This, as well as other needs, are met by the present invention which uses the RF noise voltage present on an IC package lid to determine the likely far-field EMI behavior of that IC during system testing. In particular, the voltage resulting from mutual impedence coupling of a conductor with the package lid is used to assess the EMC of the IC. The present invention produces stable and repeatable measurement data in the frequency ranges of typical electronic system emissions. Further, it produces data which, while representative of the EMI contribution of the IC, also correlates well with the far-field system test behavior of the IC.
The needs are also met by embodiments of the invention which provide a determination of EMI compliance of a system by first operating a replaceable IC component at an operating speed, measuring the noise voltage level on the lid of the IC and then calculating a system level EMI valve based on the measured noise level.
The needs are further met by embodiments of the invention which provide a test apparatus for measuring the voltage potential on a lid of an IC comprising the IC attached to the IC so that it is mutually impedance coupled with the lid and a meter connected to the conductor to measure the voltage potential.
The foregoing features, as well as other aspects and advantages, of the present invention will become more apparent from the following detailed description, claims and drawings.